Current estimates indicate that between 58% and 78% of MSM in the US acquire HIV from their main partners within the context of a same-sex relationship (e.g., male couples). Studies with male couples have examined which factors, including sexual agreements, impact their risk for acquiring HIV. Many male couples in the US form a sexual agreement: an explicit agreement made between two partners about what sexual and other behaviors may occur within and outside of their relationship. By nature, couples' sexual agreements have direct implications toward their risk for HIV and other STIs. Because few HIV prevention interventions exist for male couples, new and novel approaches that consider the relational context of male couples are urgently needed to significantly improve HIV prevention. To help fulfill this critical gap, the proposed 3-year study aims to develop and pilot theoretically derived, web-based HIV prevention toolkit for at-risk HIV-negative male couples in the US. Through the completion of modules, the interactive HIV prevention toolkit will guide and assist at-risk HIV-negative male couples who currently practice unprotected anal intercourse (UAI) - the primary sexual risk behavior for HIV acquisition among MSM - form an explicit sexual agreement that integrates interval testing for HIV and STIs and prevention messaging while uniquely meeting the needs of their relationship. The web-based prevention toolkit will consist of two components: an interactive website that contains theoretically-based modules for couples to complete while forming their agreement, and a corresponding smartphone application (app) that will contain a copy of the agreement along with a list of GPS- enabled sexual health resources, such as where they can test for HIV and STIs, and access free safer sex supplies and biomedical approaches to prevention. Development of the prevention toolkit will occur through 3 phases. In Phase 1, we will conduct individual- and couple-level interviews in Detroit and Atlanta with 30 HIV- negative male couples who practice UAI and currently have a sexual agreement (15 open, 15 closed) to obtain feedback about the content in the prevention toolkit modules, derived from our extensive previous HIV prevention studies about male couples' sexual agreements and relationship dynamics. Feedback obtained from Phase 1 will be integrated to improve the content of the modules, and used by the software company to design the first version of prevention toolkit. Phase 2 will include conducting 2 focus groups, one in Detroit and one in Atlanta, with 7 HIV-negative male couples to pilot test the content of the modules, and obtain feedback about the appeal and ascetics of the interactive exercises and components of the prevention toolkit. By integrating findings from Phase 2, further refinements will be made to the prevention toolkit before beginning to pilot test the intervention. For Phase 3, a small prospective RCT with a 6-month follow-up period will be conducted via the Internet with a national convenience sample of 160 at-risk HIV-negative male couples to determine whether couples who receive the intervention (i.e., prevention toolkit) will form and adhere to their sexua agreement, increase their testing rates for HIV and STIs, and abstain from or decrease their occurrences of UAI with casual MSM partners when compared to couples in the control group. The proposed study is innovative because technology is appropriately used to maximize reach and impact while uniquely tailoring to the needs of each couple. We anticipate our pilot intervention will show promise for reducing male couples' HIV risk, which would then be further tested for efficacy through a larger RCT.